diff --git a/library/HTMLPurifier/AttrDef/CSS/Ratio.php b/library/HTMLPurifier/AttrDef/CSS/Ratio.php
new file mode 100644
index 000000000..e08e2c496
--- /dev/null
+++ b/library/HTMLPurifier/AttrDef/CSS/Ratio.php
@@ -0,0 +1,46 @@
+<?php
+
+/**
+ * Validates a ratio as defined by the CSS spec.
+ */
+class HTMLPurifier_AttrDef_CSS_Ratio extends HTMLPurifier_AttrDef
+{
+    /**
+     * @param   string               $ratio   Ratio to validate
+     * @param   HTMLPurifier_Config  $config  Configuration options
+     * @param   HTMLPurifier_Context $context Context
+     *
+     * @return  string|boolean
+     *
+     * @warning Some contexts do not pass $config, $context. These
+     *          variables should not be used without checking HTMLPurifier_Length
+     */
+    public function validate($ratio, $config, $context)
+    {
+        $ratio = $this->parseCDATA($ratio);
+
+        $parts = explode('/', $ratio, 2);
+        $length = count($parts);
+
+        if ($length < 1 || $length > 2) {
+            return false;
+        }
+
+        $num = new \HTMLPurifier_AttrDef_CSS_Number();
+
+        if ($length === 1) {
+            return $num->validate($parts[0], $config, $context);
+        }
+
+        $num1 = $num->validate($parts[0], $config, $context);
+        $num2 = $num->validate($parts[1], $config, $context);
+
+        if ($num1 === false || $num2 === false) {
+            return false;
+        }
+
+        return $num1 . '/' . $num2;
+    }
+}
+
+// vim: et sw=4 sts=4
diff --git a/library/HTMLPurifier/CSSDefinition.php b/library/HTMLPurifier/CSSDefinition.php
index 1bc419c53..09321fd25 100644
--- a/library/HTMLPurifier/CSSDefinition.php
+++ b/library/HTMLPurifier/CSSDefinition.php
@@ -304,6 +304,13 @@ protected function doSetup($config)
                     $trusted_max_wh
                 );
 
+        $this->info['aspect-ratio'] = new HTMLPurifier_AttrDef_CSS_Multiple(
+            new HTMLPurifier_AttrDef_CSS_Composite([
+                new HTMLPurifier_AttrDef_CSS_Ratio(),
+                new HTMLPurifier_AttrDef_Enum(['auto']),
+            ])
+        );
+
         // text-decoration and related shorthands
         $this->info['text-decoration'] = new HTMLPurifier_AttrDef_CSS_TextDecoration();
 
diff --git a/tests/HTMLPurifier/AttrDef/CSS/RatioTest.php b/tests/HTMLPurifier/AttrDef/CSS/RatioTest.php
new file mode 100644
index 000000000..f1d27af5a
--- /dev/null
+++ b/tests/HTMLPurifier/AttrDef/CSS/RatioTest.php
@@ -0,0 +1,24 @@
+<?php
+
+class HTMLPurifier_AttrDef_CSS_RatioTest extends HTMLPurifier_AttrDefHarness
+{
+
+    public function test()
+    {
+        $this->def = new HTMLPurifier_AttrDef_CSS_Ratio();
+
+        $this->assertDef('1/2');
+        $this->assertDef('1 / 2', '1/2');
+        $this->assertDef('1');
+        $this->assertDef('1/0');
+        $this->assertDef('0/1');
+
+        $this->assertDef('1/2/3', false);
+        $this->assertDef('/2/3', false);
+        $this->assertDef('/12', false);
+        $this->assertDef('1/', false);
+        $this->assertDef('asdf', false);
+    }
+}
+
+// vim: et sw=4 sts=4
diff --git a/tests/HTMLPurifier/AttrDef/CSSTest.php b/tests/HTMLPurifier/AttrDef/CSSTest.php
index 2367c187f..cff044ec1 100644
--- a/tests/HTMLPurifier/AttrDef/CSSTest.php
+++ b/tests/HTMLPurifier/AttrDef/CSSTest.php
@@ -72,6 +72,10 @@ public function test()
         $this->assertDef('min-width:50rem;');
         $this->assertDef('min-width:50vw;');
         $this->assertDef('min-width:-50vw;', false);
+        $this->assertDef('aspect-ratio:16/9;');
+        $this->assertDef('aspect-ratio:auto;');
+        $this->assertDef('aspect-ratio:16/9 auto;');
+        $this->assertDef('aspect-ratio:auto 16/9;');
         $this->assertDef('text-decoration:underline;');
         $this->assertDef('text-decoration-line:overline;');
         $this->assertDef('text-decoration-style:dashed;');